Here's why:
* Chemical Reactions Continue: Once you mix hair bleach (usually a powder and a developer), a chemical reaction begins. This reaction produces oxygen and other byproducts that are essential for the bleaching process. Even in the refrigerator, this reaction will continue, although it might slow down.
* Pressure Buildup: The continued chemical reaction can create pressure within the container. This could cause the container to swell, crack, or even explode, which is extremely dangerous.
* Loss of Effectiveness: Even if the container doesn't explode, the bleach will lose its effectiveness over time as the reaction continues. It won't lighten hair as well, and you might end up with uneven or damaged results.
* Unpredictable Results: Using bleach that has been previously mixed and stored can lead to unpredictable results. The concentration of the chemicals will have changed, and you won't be able to accurately gauge its strength or processing time.
* Safety Concerns: Storing chemicals in the refrigerator near food is generally not a good idea. There's a risk of contamination or accidental ingestion.
The Rule: Always mix only the amount of bleach you need for a single application and discard any leftover mixture immediately after use. Don't try to save it for later.
If you have powder and developer, and they haven't been mixed, they can be stored in a cool, dry place, but not the refrigerator.